The copending Application, MECHANISM FOR CONSTANT BALANCE, discloses a mechanism used to balance one force against another, where the ratio between the forces varies in a predictable way. The present Invention incorporates much of the teaching of the copending Application.
It is known in the art to use springs to assist the raising of changing loads, particularly window shades and blinds. Michelman U.S. Pat. No. 2,350,286 has structural similarity to the present Invention, but does not track the changing load.
Patents disclosing spring assists arranged to track the changing load include those using fusees (tapered sheaves) to mediate between spring and load, such as Cusumano, U.S. Pat. No. 2,420,301 and Hiller et al, U.S. Pat. No. 5,133,399. Kuhar, U.S. Pat. No. 5,531,257, uses a modified "constant force" spring to provide a variable tracking force.
The copending Application describes a mechanism which preferably comprises a variable pitch screw and a nut. The preferred screw form is a twisted rectangular bar. In the present Invention, the bearing surface of the screw is expanded to reduce the force per unit area imposed on the screw material.
An alternate screw form, mentioned in the copending Application, is preferred for some embodiments of the present Invention. Particularly for custom or short-run applications, the alternate screw form is more easily manufactured.
The copending Application discloses a compact embodiment, particularly for use with roller shades. In the present Invention, the compact embodiment is modified, particularly for use with venetian blinds and other loads which increase as they are lifted. The present Invention better transfers force from the screw to the support structure in such applications.
The copending Application does not particularly address the tilt mechanism commonly associated with venetian blinds. The preferred compact embodiment of the present Invention includes a low profile tilt mechanism. The preferred tilt mechanism also allows lift cords to be run more directly to the cord winder of the Invention.
The present Invention incorporates a preferred bottom bar, preferably for venetian blinds, which allows convenient balance adjustment.
A preferred embodiment of the present Invention, particularly used with heavy loads, includes a linkage to insure parallel operation of a plurality of screw assemblies.